This invention relates to a transmission power control system and method for use in a digital mobile communication system, in particular to a reverse-link transmission power control system of the digital mobile communication system to control reverse-link transmission power.
In the digital mobile communication system, the reverse-link (or up-link) transmission power is generally controlled to save battery consumption of mobile stations with keeping a desired receiving quality at a base station which communicate with the mobile stations and to control interference between transmission signals transmitted from the mobile stations. Especially, such transmission power control is indispensable for the code division multiple access (CDMA) system because the mobile stations simultaneously uses a common frequency band in the CDMA system. The common frequency band is also used in other cells adjoining the cell covered by the base station in the CDMA system.
A conventional transmission power control system includes a part provided in the base station and other parts provided in the base stations.
The base station has a plurality of receivers for receiving reverse-link (or up-link) transmission signals transmitted from the mobile terminals. IF the number of the receivers is N (N: a natural number), the base station can receive N of the reverse-link transmission signals. When each of the receivers receives the reverse-link transmission signal transmitted from a certain one of the mobile terminals, it demodulates the reverse-link transmission signal to produce a demodulated signal.
The transmission power control system comprises signal-to-noise ratio (SNR) determining circuits connected to the receivers respectively in the base station. Each of the signal-to-noise ratio determining circuits determines a signal-to-noise ratio of the demodulated signal supplied from the receiver connected thereto. Herein the noise of the demodulated signal includes not only thermal noise but also interference. Accordingly, the signal-to-noise ratio is also called a signal-to-interference ratio (SIR) when attention is paid to the interference. Transmission power control (TPC) bit generators are connected to the SNR determining circuits respectively.
Each of the TPC bit generators generates a transmission power control (TPC) bit signal in response to the SNR determined by the SNR determining circuit connected thereto. The TPC bit signal is used to require the corresponding mobile station to increase of the transmission power when the SNR is smaller than a predetermined threshold. On the contrary, the TPC bit signal is used to require the corresponding mobile station to decrease of the transmission power when the SNR is larger than the predetermined threshold.
The base station multiplexes the TPC bit signal and a forward-link (or down-link) information signal for the corresponding mobile station to the corresponding mobile station.
When the corresponding mobile station receives the TPC bit signal and the forward-link information signal for the corresponding mobile station, it controls the transmission power in response to the TPC bit signal transmitted from the base station together with the forward-link information signal for the corresponding mobile station.
Thus, the transmission power of each mobile station is controlled so that the corresponding SNR at the base station becomes larger than a desired SNR and the transmission power becomes as small as possible.
Such a transmission power control system is disclosed in Japanese Unexamined Patent Publication (JP-A) No. 8-32515.
As mentioned above, the mobile stations use the common frequency band to communicate with the base station in the CDMA system. In addition, the common frequency band is also used in other cells adjoining the cell covered by the base station in the CDMA system. Thus, increase of the mobile stations simultaneously using the common frequency band brings increase in interference between the transmission signals used in the CDMA system. When the interference becomes large, the conventional transmission power control system goes on repeating that it produces the transmission power control bit signals which require the corresponding mobile stations to increase the transmission power. As a result, a large number of the mobile stations using the common frequency band transmit the transmission signals with the maximum transmission power. Accordingly, the interference is not suppressed, rather, becomes larger. In this situation, each of the mobile stations wastes electricity of a battery on trying to improve the SNR at the base station. In addition, the maximum number of the mobile stations, which can simultaneously use in the mobile communication system, becomes small because of the interference between the cells. In other words, a connection capacity of the mobile communication system becomes small because of the interference between the cells.